Die cast housings for electrical equipment: sealing, earthing and heat

Three jobs in one casting

An electrical enclosure does three jobs at once. It carries the mechanical load of the equipment inside, it keeps dust and water away from live parts, and it gives heat a path out. A die cast aluminium housing can do all three, but only when the casting and the machining are planned together. A housing designed as a plain box with a lid usually seals badly and bonds badly, because neither function was given a feature.

Die casting is chosen for housings because it produces a thin, stiff wall with integral bosses, ribs and mounting feet in one cycle. The trade-off is that the casting carries draft, a parting line, ejector marks and a skin whose thickness varies with the flow. Each of those interacts with the seal, the earth bond and the thermal path.

Sealing faces and gasket lands

The sealing face is the most expensive surface on the housing. It has to be flat enough for the gasket to compress evenly, smooth enough not to tear the gasket, and stiff enough not to deflect between fasteners. An as-cast face rarely meets all three. The usual route is to cast a land with 0.5 to 1 mm of machining stock and face it after casting, then drill and tap the fastener holes from the same setup.

Flatness matters more than roughness for a compressed gasket. A smooth but bowed face leaks at the ends, because the gasket is compressed at the screws and relaxed between them. A flat face with a visible tool pattern seals if the gasket is soft enough. Call out flatness across the seal path, not over the whole face, and give a roughness band. An Ra of 1.6 to 3.2 micrometres is a common starting point, but the gasket supplier's figure takes priority.

Fastener spacing sets the land width. A long unsupported span between screws lets the lid lift under pressure or vibration. As an illustrative rule, keep screw spacing below about ten times the gasket width and place a screw near every corner. A cast boss at each screw gives the fastener full thread depth and keeps the clamping force out of a thin wall.

• Groove or flat: an O-ring needs a groove with a controlled depth and width, while a flat gasket needs a wide land and a stiffer lid.
• Pressure equalisation: a fully sealed housing sees a pressure difference at every thermal cycle, so a breather element protects the seal.
• Cable entries: every gland is a second sealing interface with its own torque, thread and gasket.

Earthing is a finish decision

An earth bond is an electrical joint, not a bracket. It needs metal to metal contact, enough area, and protection from corrosion at the joint. On a die cast housing the natural oxide and the coating are both insulators, so the bond needs a dedicated pad that is machined or masked before finishing.

Anodising is the usual trap. The anodic layer is hard, decorative and electrically insulating. A housing anodised all over will not bond through the mounting screws, and the assembler may not discover this until the earth continuity test. Mask the bonding pads before anodising, or machine them after coating, and show the mask on the drawing. Paint behaves the same way. Powder coat is thicker and hides a poor casting surface, but it is still an insulator.

A star washer that bites through paint gives a connection that depends on coating thickness and torque, and it corrodes at the bite. A masked, plated pad with a defined contact area is easier to inspect. Tin plating is common on an aluminium pad, and a jointing compound or a bimetallic transition may be needed where copper meets it.

State the bond resistance limit and the test current. A common production check is a low resistance measurement between the pad and the mating earth point, with the limit taken from the product safety standard. Do not leave the limit to the assembler, because a pass on one bench and a fail on another is usually a measurement disagreement rather than a part defect.

The housing as a heat path

A die cast housing is often the only heat sink in the product. Heat leaves the power stage through a thermal pad or a machined boss, spreads into the housing wall, and leaves by convection and radiation. The path has three resistances in series: the interface at the component, the conduction through the casting, and the surface to the air.

The interface is usually the largest and the easiest to spoil. A component bolted to an as-cast surface touches at a few high points, and the gap between them is filled with air. A machined boss with a flatness call and a thin thermal pad gives a repeatable contact. Put the boss directly behind the component rather than offset by a rib, and keep the pad area within the flat region of the boss.

Cast walls are thin, so spreading resistance is real. A 2.5 mm wall carries heat sideways only a short distance before the outer surface area does the work. Where one device dissipates a large share of the total, a local thickening, a set of external fins or a copper spreader may be needed. Fins can be cast at little extra tooling cost if the draft and the die direction allow them, and fins parallel to the die opening are easier to cast than a deep blind pocket.

Four ways to seal a housing

The sealing method follows the service life and the number of times the housing will be opened. A joint that is opened for maintenance needs a gasket that can be replaced. A joint that is closed once at the factory can use a dispensed bead or potting. The table compares the four common routes on the design call each one makes.

What the drawing should carry

Show the alloy and the casting standard, the machining allowance, the sealing face flatness and roughness, the earth pad finish, the mask areas, the interface flatness and the leak test. Add the IP rating the housing is expected to meet and say whether the test is on the empty housing or the assembled unit.

State the pressure test method and the acceptance limit, or refer to the product standard. A housing described as waterproof without a test is a claim, not a specification. The same applies to the earth bond. Give the limit, the test current and the instrument, or the result will depend on who is holding the meter.

Finally, show the parting line and the ejector mark areas. A sealing face or a visible front panel should not sit on either. Moving a mark to a hidden face costs nothing at the design stage and a second operation once the die exists.